Introduction

Different definitions of the day are based on the apparent motion of the Sun across the sky (solar day). The reason for this apparent motion is the rotation of the Earth around its axis, as well as the revolution of the Earth in an orbit around the Sun.

A day as opposed to night is commonly defined as the period that sunlight reaches the ground, assuming that there are no local obstacles. Two effects make that days are on average longer than nights. The sun is not a point, but has an apparent size of about 32 minutes of arc. Additionally, the atmosphere refracts sunlight in such a way that some of it reaches the ground even when the sun is still below the horizon by about 34 minutes of arc. So the first light reaches the ground when the centre of the sun is still below the horizon by about 50 minutes of arc. The difference in time depends on the angle with which the sun rises and sets, but amounts to almost seven minutes at least.

Ancient custom has a new day start at either the rise or set of the Sun on the local horizon. The exact moment, and the interval between two sunrises or two sunsets, depends on the geographical position (longitude as well as latitude), and the time of year.

A more constant day can be defined by the Sun passing through the local meridian, which happens at local noon (upper culmination) or midnight (lower culmination). The exact moment is dependent on the geographical longitude, and to a lesser extent on the time of the year. The length of a such a day is nearly constant. This is the time as indicated by sundials.

A further improvement defines a fictitious mean Sun that moves with constant speed over the equator; the speed is the same as the average speed of the real Sun, but this removes the variation over a year as the Earth runs its orbit around the Sun.

The earth has over time had an increasingly longer day. The original length of one day, when the earth was new, is actually closer to 23 hours. This phenomenon is due to the moon which slows the Earth's rotation slowly over time. Because of the way the second is defined, the mean length of a day is now about 86400.002 seconds, and is increasing by about 2 milliseconds per century. See tidal acceleration for details.

Civil Day

For civil purposes, since the middle of the 19th century when railroads with regular schedules came into use, a common clock time has been defined for an entire region based on the mean local solar time at some central meridian. For the whole world, about 30 such time zones are defined. The main one is "world time" or UTC (Coordinated Universal Time).

The present common convention has the civil day start at midnight, which is near the time of the lower culmination of the mean Sun on the central meridian of the time zone. A day is commonly divided into 24 hours of 60 minutes of 60 seconds each.

Leap seconds

In order to keep the civil day aligned with the apparent movement of the sun, leap seconds may be inserted.

A civil clock day is typically 86400 SIseconds long, but will be 86401 s long in the event of a leap second (or possibly 86399 s in the event of a reverse leap second, but a reverse leap-second has never happened yet).

Astronomy

In astronomy also the sidereal day is used; it is ca. 3 minutes 56 seconds shorter than the solar day, and close to the actual rotation period of the Earth, as opposed to the Sun's apparent motion.

Boundaries of the day

For most diurnal animals, including Homo sapiens, the day naturally begins at dawn and ends at sunset. Humans, with our cultural norms and scientific knowledge, have surplanted Nature with several different conceptions of the day's boundaries. In Judaism and Jewish culture, the day begins at sunset, following Genesis 1:5, and lasts to the next eventide. MedievalEurope followed this tradition. Days such as Christmas Eve, Hallowe'en, and the Eve of Saint Agnes are the remnants of the older pattern when holidays began the evening before. Present common convention is for the civil day to begin at midnight, that is 0:00, and last a full twenty-four hours until 24:00.

In the United States, nights are named after the previous day, e.g. "Friday night" usually means the entire night between Friday and Saturday. This is the opposite of the Jewish pattern. This difference from the civil day often leads to confusion. Events starting at midnight are often announced as occuring the day before. TV-guides tend to list nightly programs at the previous day, although programming a VCR requires the strict logic of starting the new day at 0:00. Expressions like "today", "yesterday" and "tomorrow" become ambiguous during the night.

Validity of tickets, passes, etc. for a day or a number of days may end at midnight, or closing time, when that is earlier. However, if a service (e.g. public transport) operates from e.g. 6:00 to 1:00, the last hour may well count as being part of the previous day (also for the arrangement of the timetable). For services depending on the day ("closed on Sundays", "does not run on Fridays", etc.) there is a risk of ambiguity. As an example, for the Dutch Railways, a day ticket is valid 28 hours, from 0:00 to 4:00 the next night.